Abstrict
A filter cloth for liquid filtration is made as a long-pile fabric,
the pile threads 21 of which lie on the flow side and are flattened
against the supporting fabric during the filtration and form a deep
filter. At the time of back washing they are straightened by the
back washing and release the dirt particles. The back washing is
performed preferably by means of a suction bar 13, which at the
same time acts mechanically on the pile threads, in order to promote
an abrupt straightening of the pile threads at the time of back
washing and the flattening of the pile threads after the back washing.
Claims
What is claimed is:
1. A process for filtering liquid, comprising the steps of:
a. guiding a flow of liquid to be filtered through a filter cloth
having a supporting fabric (20) and a pile made of pile threads
(21) on the side against which the liquid flows; said flow of liquid
causing the pile threads (21) to be flattened toward the supporting
fabric (20) and to form a spacial filter having angled and multiply
curved flow channels;
b. back washing the filter cloth at time intervals by moving a
suction bar (13) having a suction slit (17) over said filter cloth
on the pile thread side thereof and causing the suction bar to exert
a suction force to create a back washing flow by sucking the liquid
through the filter cloth; said back washing flow causing the pile
threads (21) in the area of the suction slit (17) to be straightened
up to a position directed away from the supporting fabric (20);
c. subjecting said pile threads (21), prior to entering the suction
slit, to a mechanical retaining action which retains the pile threads
(21) in their flattened position against the fabric and prevents
them from straightening up; and
d. abruptly releasing the pile threads (21) from said mechanical
retaining action upon partial entry thereof into the suction slit
whereby the pile threads (21) are caused to straighten abruptly
within the suction slit.
2. The process of claim 1, wherein the width of the suction slit
is a multiple of the length of the pile threads (21).
3. The process of claim 1, comprising the further step of subjecting
the pile threads (21) leaving a suction slit to a mechanical flattening
action to return the pile threads (21) to their flattened position
as they leave the suction slit.
4. A device for filtering liquid comprising a filter cloth located
in the path of flow of the liquid to be filtered, the filter cloth
having a support fabric and a pile made of pile threads (21) on
its side exposed to the flow, the length, stiffness, and density
of the pile threads being controlled so that they are flattened
by the flow of the liquid being filtered and form a deep filter
with angled and multiply curved flow channels,
a suction bar (13) on the pile side of the filter cloth, with a
suction slit (17) for sucking a back washing flow through the filter
cloth at a flow rate such that in the back washing the pile threads
straighten out into a position directed away from the filter cloth,
and means for creating relative motion between the suction bar
and the filter cloth (6) in a direction of motion parallel to the
plane of the filter cloth, the suction bar (13) having a surface
(18) turned toward the filter cloth (6) at least on the side of
the suction slit (17) lying upstream with respect to the relative
motion, which surface is spaced from the filter cloth (6) by such
a short distance that it acts mechanically on the pile threads (21)
prior to entering the suction slot to retain the pile threads in
their flattened position against the fabric and abruptly release
the pile threads upon entry into the suction slit.
5. The device of claim 4, wherein the surface (18) forms a sharp
edge (19) on the edge of the suction slit (17).
6. The device of claim 4, wherein the width (3) of the suction
slit (17) is greater than the length of the pile threads (21).
7. The device of claim 6, wherein the width (8) of the suction
slit (17) amounts to a multiple of the length of the pile threads
(21).
8. The device of claim 4, wherein the pile threads (21) consist
of multifilaments with free ends separated into individual filaments.
Description BACKGROUND OF THE INVENTION
The present invention relates generally to liquid filtration by
means of a filter cloth. In particular the present invention relates
to the filtration of waste water which discharges from the final
purification stage of a waste water purification plant.
Filtering devices for liquids operating with filter cloths are
known in different designs, e.g. in the form of filter presses,
disk filters (EP-B-0 413 178) or drum filters. The invention is
applicable in the case of filtering devices of any design. The filter
cloths used in such devices are single-layer fabrics which behave
like a sieve, the holes of which are defined by the mesh of the
fabric. However, as a rule filter cloths have a multi-layer structure
comprising a supporting fabric with a relatively large mesh and
a filter-active layer. In the filler-active layer the threads or
filaments lie as multiple plies over one another. A deep or spatial
filter of this type has angled and multiply curved flow channels
within which solid particles, which are smaller than the free cross-section
of the spaces between the threads, can be held back, partially by
mechanical blocking in the flow channels and partially by adsorption
on the walls thereof. Such a filter-active layer with a deep filter
effect as a rule is made as a needle felt or tangled fiber fleece.
The flow resistance of such a filter cloth is relatively high and
increases quickly with increasing blockage of the flow channels
in the filter layer. Therefore the filter cloth must be cleaned
at relatively short intervals. As a rule this is performed by back-rinsing,
that is, by passing liquid, such as, e.g. fresh water or filtrate,
back through the filter cloth in the direction opposite to the direction
of filtering. If the side of the filter cloth against which the
liquid flows in the filtering operation is designated as the front
side and the other side as the back side, then the back washing
flow necessary for back washing the filter cloth can be created
by sucking liquid on the front side or supplying liquid under pressure
on the back side of the filter cloth, or both. The back washing
can be carried out on the entire surface of the filter cloth simultaneously
or only on a partial area of the filter cloth, which is moved back
and forth over the entire surface thereof. An example of a suction
device movable over the filter cloth for back washing is described
in the already mentioned EP-B-0 413 178.
Those solid particles, which are deposited on the outside of the
filter cloth at the time of filtering and form a layer of the pre-coat
layer type which supports the filter action, can easily be removed
by back washing. However, those solid particles which are caught
and blocked in the numerous narrow and angled flow channels of the
filter-active layer of the filter cloth, produce considerable problems
at the time of back washing. These problems are greater the more
the layer has the nature of a deep filter. In the twisting flow
channels of the filter-active layer no flow speed can be applied
which is so great that the solid particles are completely released
and washed away. Limits are set on the increase in the pressure
or, as the case may be, on the flow speed since the filter cloth
is damaged at too high a speed. The filter cloths known up to now
where the effective filter-active layer is made as a deep filter,
e.g. as needle felt or tangled thread fleece, cannot be cleaned
without residue remaining therein. These become clogged so that
they have to be replaced after a relatively short operating time.
More advantageous is the formation of the filter cloth as a so-called
pile material, that is, a pad, plush, or velvet fabric, which consists
of a relatively large-meshed supporting fabric and, for example,
a pile made of pile threads worked into the W binding. In the filter
operation the pile threads lie on the upstream side of the filter
cloth and their length, stiffness, and thickness are selected so
that the pile threads are shifted by the flowing liquid into a position
approximately parallel to the supporting fabric. The pile threads
lie in multiple layers or plies over one another and form a deep
filter containing numerous narrow, angled, and multiply curved flow
channels between the pile threads lying over one another. Solid
particles, smaller than the unobstructed width of the flow channels,
can be held therein partially mechanically and partially by adsorption.
On the other hand, at the time of the back washing the pile threads
are exposed to a liquid flow directed away from the supporting fabric
and in this way are raised relative to the supporting fabric. Then
they are largely parallel to one another and to the direction of
flow, by which the volume of the pile layer is greatly enlarged
and the flow channels existing between the pile threads are opened
and leveled. By means of this, on the one hand, the flow resistance
for the back washing liquid is reduced so that it can flow between
the pile threads at a high rate, and on the other hand the solid
particles mechanically blocked previously in the flow channels are
released, so that they can be loosened and washed away by the back
washing liquid, overcoming the adsorption forces.
Processes and devices for filtering liquids, with the use of a
pile material as a filter medium and with the use of the previously
described straightening effect at the time of back washing are known
from FR-A-2 366 864 and NL-A-81 03 750. In the case of the process
and the device in accordance with NL-A 81 03 750 it is known that
the back washing which causes the straightening of the pile threads
is created by means of a suction bar with a suction slit, which
stands opposite the pile side of the filter cloth at a short distance
and is moved parallel to this.
SUMMARY OF THE INVENTION
The object of the invention is to improve a process and a device
for liquid filtration of the specified known type so that a more
effective and more thorough cleaning of the filter cloth during
back washing is made possible. This is achieved in accordance with
the invention by means of the filtering process and the filtering
device of the present invention.
In accordance with the invention it was found that it is advantageous
to block the pile threads of the filter cloth by mechanical action
of the suction bar from straightening up until they have reached
the area of the suction slit and can straighten up abruptly in that
area. In this way the
loosening and releasing of the solid particles deposited in the
pile layer of the filter cloth is promoted.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in greater detail below by means of
embodiments with reference to the drawings. Here:
FIG. 1 shows a simplified schematic sectional representation of
the fabric structure of a filter cloth in accordance with the invention,
FIG. 2 shows the filter cloth during the filtering operation in
a schematic representation similar to that of FIG. 1,
FIG. 3 shows a detail of the filter cloth in a magnified schematic
representation,
FIG. 4 shows a schematic longitudinal section of a filtering device
made as a sieve filter,
FIG. 5 shows a schematic detail of the filtering device in accordance
with FIG. 4.
DESCRIPTION OF A PREFERRED EMBODIMENT
The structure of the filter cloth in accordance with the invention,
as shown very schematically and simplified in FIG. 1, consists of
a supporting or base fabric 20 and pile threads 21 incorporated
into the supporting fabric 20. The threads extend from the supporting
fabric 20 to one side and form a pile of the filter cloth. The supporting
fabric 20 is shown simplified with warp and weft threads. However,
the expression "supporting fabric" also is to include
flat textile articles which are not produced by weaving, but, e.g.
by knitting or in a mixed technique. The mesh width or size of the
supporting fabric 20 preferably is larger than the particle size
of the solid particles to be filtered out of the liquid. The pile
threads 21 are incorporated into the supporting fabric 20 in any
way and manner known in the art.
Preferably both the supporting fabric and the pile threads consist
of plastic filaments, the smooth surface of which promotes the purification.
At least the pile threads 21 are multifilaments composed of monofilaments,
which at least in the area of their free ends 22 can fan out into
their individual filaments, as is shown schematically in FIG. 3.
The threads of the supporting fabric 20 also can be multifilaments.
Preferably polyamide or polyester are used as the material for the
supporting fabric 20 and pile threads 21. The binding of the pile
threads 21 to the supporting fabric 20 can be reinforced by partial
welding, bonding and/or gluing.
In the following description for the sake of simplicity the side
of the filter cloth having the pile threads 21 (the left side in
FIG. 1) is designated as front side and the other side as back side.
During the filtering the filter cloth is used so that the liquid
to be filtered flows toward and against its front side, that is,
against the pile threads 21. The pile threads 21 are displaced,
or as the case may be laid flat, in the direction toward the supporting
fabric 20 by the arriving liquid to be filtered, so that they assume
the position indicated schematically in FIG. 2, in which position
adjacent pile threads 21 lie in several layers over one another
and form a filtering layer of the type of a needle felt or tangled
fiber fleece, that is, a deep filter, which forms numerous narrow,
long, and angled flow channels. An appropriate choice of the length,
stiffness, and density of the pile threads 21 is important for this
desired behavior. These parameters are mutually dependent on one
another, so that e.g. thicker, or as the case may be stiffer pile
threads 21 require a greater length. The length of the pile threads
21 as a rule is at least 10 mm, preferably more than 12 mm. Shorter
pile threads are either too stiff or do not produce the desired
"layer density", or spatial depth of the filter-active
layer in the filter operation in the position shown in FIG. 2.
At the time of back washing the filter cloth, wash water, e.g.
fresh water or filtrate, is directed from the back side (right side
in FIG. 1 and FIG. 2) through the filter cloth and at sufficient
speed so that the displaced pile threads 21 are straightened and
assume the position shown schematically in FIG. 1. In this case
the angled channels disappear between the pile threads and make
an area with a large free cross-section, by which the dirt particles
are washed away without hindrance or redepositing. In the case of
a sufficiently great flow rate of the back washing liquid, the turbulence
of the back washing can even cause a flattening or "shaking
out" of the pile threads, by means of which the release of
the dirt particles also is promoted. Experiments have shown that
a practically complete cleaning of the filter cloth can be achieved
by back washing in this way, in particular when the meshes of the
supporting fabric 20 are made so large that no solid particles can
be found in the supporting fabric 20 itself.
There is no danger of a gradual loading of the filter cloth, because
its filter-active layer formed by the pile threads can be cleaned
completely at any time by the straightening effect and the volume
increase at the time of the back washing. In the filter operation
the filter performance of the filter cloths in accordance with the
invention is completely equivalent to that of commercial filter
cloths made of e.g. needle felt or tangled fiber fleece. Experiments
with a suspension discharging out of the final purification after
a biological stage with phosphate elimination has shown that this
suspension with a solid content of e.g. 20 mg/l could be purified
in the inlet of the filter up to a maximum residual solid content
of 4 mg/l in the filtrate. With this the filter cloth in accordance
with the invention is excellently suited for maintaining the legal
provisions for phosphate elimination in the third purification stage
of a waste water purification unit.
Because of the effective back washing process the required amount
of back washing liquid is very small. Experiments have shown that
the amount of back washing liquid required for a perfectly satisfactory
cleaning is only on the order of magnitude of 0.5% to max. 2% of
the throughput of the liquid to be filtered.
As already mentioned, the filter cloth in accordance with the invention
can be used in filtering devices of any design, e.g. filter presses,
disk filters, drum filters, and the like. As a rule no design changes
in the filtering devices are necessary for the use of the filter
cloth in accordance with the invention, that is, the existing filtering
machine can be re-equipped with the filter cloths in accordance
with the invention.
A filtering device made as a disk filter is shown schematically
in FIG. 4 as a specific embodiment. Preferably it can be used for
purification of waste water which was pretreated biologically and
then subjected to a chemical handling for precipitating out dissolved
contaminants, e.g. phosphate. The pretreated waste water reaches
a basin 4 via an inlet 3, in which basin there are a number of disk-shaped
filter pieces 5, which in each case, are covered with filter cloths,
on a rotating shaft 7. The water flows through the filter cloths
6 into the interior of the disk-shaped filter pieces 5 and is removed
to the drain 8 via the rotating shaft 7. A motor 9 causes the rotation
of the shaft 7 with disk-shaped filter pieces 5 via a chain 10 and
a pinion 11. Each filter piece 5 consists of a carrying frame (not
shown), which is covered with the filter cloth 6 so that the pile
threads of the filter cloth lie on the outside of the disk-shaped
filter piece 5. Each disk-shaped filter piece 5 also can be composed
of several sectors, which in each case are covered with a sack-like
filter cloth individually, as described in EP-B-0 413 178.
The back washing of the filter cloths 6 is performed by sucking
liquid on the front side of each filter cloth 6, that is, on the
outside of each disk-shaped filter piece 5. FIG. 4 schematically
shows a suction bar 13, which stands opposite the first disk-shaped
filter element separated by a short distance, and is connected to
a suction device 14. The suction bar 13 has a slit-shaped opening
turned toward the filter piece 5, and by turning the shaft 7 the
filter element 5 is moved past the slit-shaped opening, so that
gradually the entire surface thereof can be cleaned by back washing.
If desired this back washing can be carried out continuously during
the running filter operation, or with interrupted filter operation.
If only one single suction bar 13 is provided, this must be portable
with a suitable device in such a way that it can be brought one
after the other into operating positions opposite each of the two
faces of each of the disk-shaped filter pieces 5. Alternatively
a number of stationary suction bars 13 can be provided, which in
each case are associated with one of the two face sides of the individual
disk-shaped filter elements 5.
As indicated schematically in FIG. 5, one or the other of the suction
bars 13 stands opposite the front side of the filter cloth 6 provided
with the pile threads 21 at such a short distance that it acts mechanically
on the pile threads at the same time. If it is assumed that the
filter cloth 6 moves in the direction of the arrow 16 relative to
the suction bar 13 (therefore upwardly in FIG. 5), the pile threads
21 one after the other reach the area of the suction slit 17 of
the suction beam 13. Before the pile threads 21 reach the slit 17,
they are prevented from straightening up by the lower area 18, shown
in FIG. 5, of the wall of the suction bar facing toward them. As
soon as the pile threads 21 reach the slit 17, they are straightened
up abruptly by the flow 18 sucked through from the suction bar 13
through the filter cloth 6. A tapered or sharp-edged formation of
the edge 19 of the suction slit 17 favors the abrupt straightening.
The mechanical action of the suction bar 13 on the pile threads
21 of the filter cloth 6 preferably should be such that the pile
threads 21 reaching the area of the slit 17 at the time of the transition
from the displaced state to the straightened state are spread apart,
if possible up to the supporting fabric 20, that is, they are "parted".
A profiling of the side of the suction bar 13 turned toward the
filter cloth 6, in a given case with additional projections, can
be easily determined by simple experiments. The trailing edge 25
of the suction slit 17 does not need to be sharp-edged but can e.g.
also be rounded in order to favor a gentle displacement of cleaned
pile threads. However, also conceivable are arrangements, in the
case of which the filter cloth 6 is passed back and forth, e.g.
in the pilger step, on the suction bar 13; in this case a symmetrical
configuration of the suction beam 13 to the center line of the suction
slit 17 is advantageous.
The back washing by suction can be supported by additional supply
of back washing liquid and pressure from the back side of the filter
cloth. If the construction of the filtering device permits this
(in the case of the disk filter in accordance with FIG. 4 this ought
not be the case), then a spraying bar 27 is located on the back
side of the filter cloth 6 and is moved together with the suction
bar 13, which creates a back washing jet directed to the back side
of the filter cloth through a slit opening or a number of individual
openings. This would be conceivable e.g. in the case of drum filters,
in which case the spray bars 27 can be located fixed in the interior
and the suction bar 13 on the outside of a turning drum, with the
drum covered with the filter cloth 6.
The back washing process can be supported by additional measures
known in the art. For example, the back washing liquid (fresh water
or filtrate) also can be sucked in order to loosen encrusted dirt
particles. In order to prevent an undesirable holding of dirt particles
because of static charging, the threads of the filter cloth, but
at least pile threads thereof, can be provided with known anti-static
measures, e.g. by means of metal coating, depositing of conducting
metal or carbon particle or chemical anti-static treatment.
As mentioned initially, the invention is applicable in the case
of filtering devices of any design. According to the type of the
filtering process, the means for creating the necessary relative
motion between the suction bar and the filter cloth are developed
in the direction of motion parallel to the filter cloth. This can
take place not only as in the case of the embodiment described,
by motion of the filter cloth with a stationary suction bar, but
also by motion of the suction bar.
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